The present invention relates to light emitting devices and, more particularly, to semiconductor light emitting devices that include recipient luminophoric mediums. As used herein, the term “semiconductor light emitting device” refers to a light emitting device that includes at least one light emitting diode (“LED”). Example semiconductor light emitting devices include packaged lamps, light bulbs, streetlights, downlights, ceiling-mounted troffers or various other light fixtures that include one or more LEDs.
A wide variety of light emitting devices are known in the art including, for example, incandescent light bulbs, fluorescent lights and semiconductor light emitting devices that include one or more LEDs. LEDs generally include a series of semiconductor layers that may be epitaxially grown on a substrate such as, for example, a sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide substrate. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. As the electrons and holes flow toward each other, some of the electrons will “collide” with corresponding holes and recombine. Each time this occurs, a photon of light is emitted, which is how LEDs generate light. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the region of the device where the electrons and holes recombine.
Most LEDs (chips/die) are nearly monochromatic light sources that appear to emit light having a single color (usually blue). Thus, the spectral power distribution of the light emitted by most LEDs is tightly centered about a “peak” wavelength, which is the wavelength where the spectral power distribution or “emission spectrum” of the LED reaches its maximum as detected by a photo-detector. The “spectral width” (i.e., the power of the emission as a function of wavelength) of the spectral power distribution of most LEDs is between about 10 nm and 30 nm, where the spectral width is measured at half the maximum illumination on each side of the emission spectrum (this spectral width is referred to as the full-width-half-maximum or “FWHM” spectral width).
White light is desired for many applications, and particularly for applications in which incandescent and/or fluorescent bulbs (both of which emit white light) have traditionally been used. Semiconductor light emitting devices are now widely available that produce white light. One technique for generating white light using almost monochromatic LEDs is to provide a device that includes several LEDs that each emit a light of a different color. The different colored light emitted by the LEDs may combine to produce light that is white or near-white in color. For example, by simultaneously energizing red, green and blue LEDs, the resulting combined light may appear white, or nearly white, depending on, for example, the relative intensities, peak wavelengths and spectral power distributions of the source red, green and blue LEDs.
White light may also be produced by surrounding a single blue or ultraviolet LED with one or more luminescent materials such as phosphors that convert some of the light emitted by the LED to light of one or more other colors. The combination of the light emitted by the LED that is not converted by the luminescent material(s) and the light of other colors that are emitted by the luminescent material(s) may produce a white or near-white light.
As an example, a white light emitting semiconductor light emitting device may be formed by coating a gallium nitride-based blue LED (i.e., an LED that emits light having a peak wavelength in the blue color range as defined herein) with a yellow luminescent material such as a cerium-doped yttrium aluminum garnet phosphor, which has the chemical formula Y3Al5O12:Ce, and is commonly referred to as YAG:Ce. The blue LED emits light having an emission with a peak wavelength of, for example, between 440-450 nm. Some of blue light emitted by the LED passes between and/or through the YAG:Ce phosphor particles without being down-converted, while other of the blue light emitted by the LED is absorbed by the YAG:Ce phosphor, which becomes excited and emits light with a peak wavelength of about 550 nm and a FWHM spectral width of about 115 nm. The combination of blue light emitted by the LED that is not converted by the phosphor along with the green, yellow, orange and red light emitted by the YAG:Ce phosphor may appear white to an observer. Such light is typically perceived as being cool white in color, as it is primarily comprises light on the lower half (shorter wavelength side) of the visible emission spectrum. To make the emitted white light appear more “warm” and/or exhibit better color rendering properties, red-light emitting luminescent materials such as Eu2+ doped CaAlSiN3 based phosphor particles may be added to the coating applied to the blue LED.
The above-described approaches for generating white light may also be combined. For example, semiconductor light emitting devices are widely available that include one or more blue LEDs that are coated with a phosphor that emits light having a peak wavelength in the yellow or green color ranges along with one or more additional LEDs that emit light having a peak wavelength in the red color range.
In general, luminescent materials may absorb light having first wavelengths and re-emit light having second wavelengths that are different from the first wavelengths. For example, “down-conversion” luminescent materials may absorb light having shorter wavelengths and re-emit light having longer wavelengths. Phosphors are one type of luminescent material that are widely used to convert a single-color (typically blue or violet) LED into a broader white light spectrum. However, it will be appreciated that other luminescent materials may be used that absorb light at one wavelength and re-emit light at a different wavelength in the visible spectrum such as nanophosphors, quantum dots, scintillators, day glow tapes, and inks that glow in the visible spectrum upon illumination with (e.g., ultraviolet) light.
A medium that includes one or more luminescent materials that is positioned to receive light that is emitted by an LED or other semiconductor light emitting device is referred to herein as a “recipient luminophoric medium.” Exemplary recipient luminophoric mediums include layers having luminescent materials that are coated or sprayed directly onto a semiconductor light emitting device or on surfaces of the packaging thereof, and clear encapsulants (e.g., epoxy-based or silicone-based curable resin) that include luminescent materials that are arranged to partially or fully cover a semiconductor light emitting device. A recipient luminophoric medium may include one medium layer or the like in which one or more luminescent materials are mixed, multiple stacked layers or mediums, each of which may include one or more of the same or different luminescent materials, and/or multiple spaced apart layers or mediums, each of which may include the same or different luminescent materials.